1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor structure comprising clusters and/or nanocrystals of silicon which are present in distributed form and a matrix of a silicon compound and to a semiconductor structure of this kind.
2. Statement of the Prior Art
It is known that Si clusters or nanocrystals embedded in SiO2 show a strong visible luminescence.
There is currently much interest in designing semiconductor structures based on silicon which are able to emit light and which are suitable for integration into optoelectronic circuits in chip form, for example in the form of lasers and high speed telecommunication devices, and for use in memories.
The Nov. 23, 2000 issue of Nature, Volume 408 includes on pages 411 and 412 a general article by Leigh Canham giving a general overview of the concept of obtaining light from silicon with particular reference to silicon nanocrystals. The same edition of nature also contains, on pages 440 to 444 an article by L. Pavesi and colleagues entitled “Optical Gain in Silicon Nanocrystals”.
A general discussion of the physics of crystallization of amorphous superlattices in the limit of ultra thin films with oxide interfaces is given in an article by M. Zacharias and P. Streitenberger in Physical Review B, Volume 62, No. 12 of Sep. 15, 2000 on pages 8391 to 8396.
At this stage it should be explained that silicon nanocrystals are crystals of silicon with dimensions in the nanometer (nm) range. The nanocrystals contain relatively few silicon atoms and have properties which differ from those of larger silicon crystals. Accumulations of silicon atoms without crystallization are sometimes also referred to as clusters.
To date there have been two principal proposals for the generation of such silicon clusters or nanocrystals. The first proposal is, e.g., described in the article by L. Pavesi et al in the named issue of Nature. This article describes how silicon ions have been implanted by negative ion implantation techniques into ultra-pure quartz substrates or into thermally grown silicon dioxide layers on Si substrates followed by high temperature thermal annealing, for example at 1100° C. for one hour. This heat treatment allows the implanted silicon atoms to move within the substrate and form Si clusters or nanocrystals during the high temperature thermal annealing. In the cited article by Pavesi et al, it is stated that the silicon nanocrystals embedded within silicon dioxide matrix are of the order of 3 nm in diameter in concentration of 2×1019 cm−3.
A similar process of forming Si nanocrystals by ion implantation is described in an article by W. Eckstein and colleagues entitled “Modeling of the Formation and Properties of Nanocrystals in Insulator Matrices (SiO2:Si, ZrO2(Y):Zr) Produced by Ion Implantation” which has been published in the Proceedings of the International Conference on Ion Implantation Technology, 17–22 September 2000, pages 757–760.
Although ion implantation can be used to produce silicon nanocrystals it has the significant disadvantage that large area ion implantation with a high Si ion dose is not regularly used in silicon electronic production systems and thus the need to use ion implantation is a substantial complication of the manufacturing process.
A second proposal for the manufacture of silicon nanocrystals is to be found in U.S. Pat. No. 6,060,743. This U.S. patent describes a variety of semiconductor structures all of which basically involve the deposition of a thin amorphous silicon layer on a silicon dioxide film. The amorphous silicon layer is for example just 1 nm thick. The thin silicon film is deposited at a relatively low temperature and is subsequently heated to about 800° C. without being exposed to the atmosphere (in order to prevent oxidation). The heating caused an agglomeration phenomenon in the flat amorphous silicon layer formed on a silicon oxide film. As a result, the amorphous silicon layer is converted into independent crystals of about 10 nm in diameter at most and about 5 nm in height with a density of the silicon nanocrystals of 3.5×1011 cm−3. The silicon nanocrystals are formed on the silicon dioxide film of the silicon substrate. Thereafter a further silicon dioxide film is deposited over the substrate and the nanocrystals.
A similar process of forming Si clusters in amorphous SiO2 is described in an article by Y. Liang and colleagues entitled “The Mechanism of Formation and Photoluminescence of Si Quantum Dots Embedded in Amorphous SiO2 Matrix” which has been published in the Proceedings of the Fourth International Conference on Thin Film Physics and Applications, Shanghai, China, 8–11 May 2000, vol. 4086, pages 174–177.
This method has the disadvantage that only a relatively small density of nanocrystals can be achieved and that the method can only be rationally extended to the production of a few “layers” of nanocrystals because of the process that is used.